Pluto's wobbly moons dance to a chaotic tune

This set of computer modeling illustrations of Pluto’s moon Nix shows how the orientation of the moon changes unpredictably as it orbits the “double planet” Pluto-Charon(Credit: NASA/ESA/M. Showalter (SETI)/G. Bacon (STScI))

Pluto and its largest satellite Charon are an odd pair. The relative size of the Charon means the two act as a double planet system, orbiting a center of gravity located between them in space. A study of data gathered by NASA's Hubble Space Telescope has now found that this interaction is having an odd effect on Pluto's smaller moons, causing them to tumble unpredictably with no apparent method to their madness.

Scientists made the discovery after analyzing images of Pluto taken by Hubble between 2005 and 2012. Unpredictable changes in the light bouncing off Nix and Hydra, two of Pluto's smaller moons, hinted that something strange was occurring in the planet's system. Comparing these variations in brightness to models of bodies spinning in complicated gravitational fields, the scientists were able to establish the wobbly nature of the moon's movements.

This is a result of the shifting gravitational field caused by Charon and Pluto's unorthodox relationship. As they spin around one another, their changing gravitational fields cause the smaller moons caught up in the whole kerfuffle to make irregular movements. The moon's egg-like, rather than spherical shape, further complicates these tumbles.

"Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm," says John Grunsfeld, associate administrator of NASA’s Science Mission Directorate in Washington. "When the New Horizons spacecraft flies through the Pluto system in July we’ll get a chance to see what these moons look like up close and personal."

Among the many questions scientists hope the New Horizons probe will answer relates to a newfound uncertainty surrounding the color of the moons. The Hubble data also revealed that Kerberos, discovered in 2011 and thought to be the smallest of Pluto's moons, is dark, while the other icy moons shine quite brightly. It had been thought that dust showers resulting from meteorite impacts covered all the moons, making Kerberos something of a black sheep of the family.

The New Horizons spacecraft is not set to rendezvous with Pluto until July 14, but has already returned valuable insights into the nature of the planetary system. After a 9.5 year journey, the probe sent back images of its first look at Pluto and Charon in February. It then captured the first colored photo of Pluto taken by a spacecraft on approach and snapped all of its moons together for the first time in May.

The chaotic nature of the Pluto-Charon relationship could offer clues as to how other binary systems function, such as the worlds orbiting double stars that have been discovered by the Kepler Space Telescope since 2009.